Salix

The flower of a willow consists of a single pistil or (1–) 2–7 (occasionally more) stamens apparently in the axil of a small scale or bract. These flowers are grouped in “catkins” or aments. Since the flowers are small and lack a perianth, the species are more difficult to describe than in many genera, with the added complications that the sexes are on separate plants and that in many species the flowers appear before the leaves. In addition, hybridization is not uncommon among certain species, so that the willows have a reputation for being unusually difficult to identify.

The general key below includes all species, and is based on characteristics of the flowers, fruit, and leaves. It includes fairly full descriptions of the species insofar as they differ from each other, and is intended to work with either staminate or pistillate plants, as long as leaves are present. In addition, supplementary keys are provided for staminate and pistillate plants of those species that may be found (at least sometimes) with the flowers or fruit well developed before the leaves appear. These keys will be helpful in many cases, but additional leaf collections from the same individual plant should be made for certainty.

Identification of willows requires patience and good magnification of about 10× with adequate illumination. In several species, the presence or absence of copper-colored hairs on young foliage is a very helpful characteristic; do not assume that these are absent until a careful search has been made. Similarly, examine the petioles of a number of leaves before concluding that glands are absent. Look at several leaves to gain an impression of shape and the nature of the margin. In short, do not base decisions on a single examination of a single part. In a few species, the sequence of flowering in the ament is distinctive. This is a characteristic best seen in partly developed staminate aments; fully mature or exceedingly young aments will not demonstrate the sequence. The filaments of older stamens may shrivel and shorten, making them falsely appear younger, so great caution must be exercised in using this character. In S. discolor, S. humilis, S. myricoides, and S. cordata the first well-developed stamens with mature anthers appear at the apex of the ament, those at the base developing subsequently.

Salix babylonica L., Weeping Willow, has been reported from the state, but is barely hardy quite this far north. True S. babylonica has capsules shorter than either S. alba or S. euxina, and the leaves are finely toothed. Most if not all Weeping Willows in this region are actually more hardy hybrids of S. babylonica with S. euxina (S. ×pendulina Wenderoth) and S. alba (S. ×sepulcralis Simonk.), if not simply cultivars of S. alba var. tristis. We have few documented records of any Weeping Willow (i.e., one with long pendulous branches) escaping from cultivation in Michigan, though they may persist long after cultivation, but we have seen a very few collections of both S. ×pendulina and S. ×sepulcralis apparently growing outside of cultivation.

3. Leaf blades closely serrate (the teeth on mature leaves usually much less than 3 mm apart), narrowly lanceolate to ovate or elliptic (except in S. nigra, with 4–7 stamens); stipules, at least on shoots, conspicuous in some species; stamens more than 2 or the filaments nearly or quite glabrous; capsules glabrous.

4. Blades linear-lanceolate, acute to somewhat rounded at base, at maturity mostly 4–10 times as long as wide; stamens 4–7; petioles usually not glandular at the summit; capsules 3–4 (–4.8) mm long, with styles less than 0.3 mm long (or rarely slightly longer).

6. Unfolding young leaves usually very reddish, moderately to very sparsely pubescent; older leaves usually somewhat whitened beneath, glabrous or nearly so, the marginal teeth tending to be crenulate and with glands absent or very small and ± sunken; stalks 1–2.5 mm long, usually about equaling or exceeding the scales; styles ca. 0.5 mm long or shorter; young aments flowering from base to apex; year-old branchlets densely to sparsely (and in patches) puberulent, rarely somewhat villous.

7. Margins of foliage leaves (not always peduncular bracts) entire or sometimes (especially on sprouts) rather coarsely and irregularly or obscurely crenulate-serrate on apical half, often ± revolute; stamens 2 (or 1 in S. purpurea); capsules pubescent (except in S. pedicellaris and S. myricoides) [Note: All species included under this lead, except S. pedicellaris, are also in the supplementary keys to flowering specimens; those keys may be helpful to consult for material with aments but with leaves not yet fully mature].

10. Aments at maturity 3–9 cm long (or the staminate, and rarely pistillate, as short as 2 cm), on short green-bracted branchlets; capsules yellow-brown to greenish; styles usually 0.6–1.5 mm long; anthers 0.6–1 mm long; leaves ± acute at apex, with at least a few obscure teeth; branchlets, especially of current year, usually ± densely pubescent; stipules often present.

9. Aments essentially sessile, at most with tiny green or brown bracts at base; capsules pubescent, the stalks various; filaments often pilose at base; anthers yellow to red or purplish; axillary buds up to 13 mm long on sprouts (though often under 5 mm, especially in S. discolor and S. planifolia); leaves ± obscurely toothed on about the apical third or half.

13. Pubescence of leaves consisting of ± curled, tomentose, or woolly hairs (at most silky on youngest leaves just emerging from bud).

15. Under surface of leaf blade actually green, hidden by dense white- or gray-tomentose pubescence (rarely glaucous with sparser white tomentum); blades linear to lanceolate or narrowly oblong, (4–) 5–12 (–17) times as long as wide; ovary and capsule ± densely white-tomentose, on a stalk up to 1 mm long; young branchlets usually with flocculent tomentum.

15. Under surface of blades ± whitened or glaucous, seldom completely hidden by the pubescence; blades generally broader, usually less than 5 times as long as wide (rarely up to 6 times); ovary and capsule silky-pubescent, on a stalk 1–6 mm long; young branchlets glabrous or puberulent to villous (not flocculent-tomentose).

16. Aments on short leafy branchlets; pubescence (even on young reddish leaves) whitish, without admixture of red or coppery hairs; leaves usually retaining at least sparse pubescence throughout above when fully developed, rugose, with veins and veinlets impressed above and prominent beneath; scales of aments rather narrow, whitish to pale brown, of uniform color or slightly darker at base or pink-tipped.

16. Aments on short branchlets or sessile, but only rarely leafy-bracted (hybrids?); pubescence of unfolding leaves (especially above) in small or large part of copper-colored hairs; leaves becoming glabrous above when fully developed, except often for whitish or coppery hairs on midrib, flat or (S. humilis) slightly rugose (a few coppery hairs occasionally present at maturity); scales of aments typically rather broad, ± obovate, very dark brown to black except for pale base.

17. Year-old branchlets pubescent or puberulent at least in small patches above the nodes [Note: Vegetative specimens not safely distinguished; see text. If old aments are present, the following may help].

7. Margins of leaves (at least mature foliage leaves) ± finely and distinctly serrate (the teeth rather distant in S. exigua and S. euxina) almost or quite to the base of the blade; stamens 2 to several; capsule pubescent or glabrous.

22. Petiole with prominent (sometimes stalked) glands or projections above, at, or near junction with the blade.

25. Blades of mature leaves ca. 1.5–3.5 times as long as wide, green or slightly whitened beneath, acute to short-acuminate; petioles and young foliage glabrous; stipules usually present (falling early) on young sprouts; leaves or leafy bracts on the short flowering branchlets usually entire or nearly so; scales, especially in staminate aments, sparsely pubescent to glabrous except at base; capsules smoothish and 4–6 mm long; uncommon introduced large shrub or small tree.

25. Blades of mature leaves 2–6 times as long as wide, green to strongly glaucous beneath; leafy bracts of flowering branchlets finely toothed, like the foliage leaves; plants common native shrubs of ± natural habitats. [The two following species differing from S. pentandra as described below; e.g., if leaves whitened beneath, then stipules always lacking, capsules larger, and scales pubescent to their tips; if green beneath, then tending to be long-acuminate on sprouts, often pubescent on petioles when young, the capsules ripening early].

26. Leaves dark to pale green but not whitened beneath, long-acuminate at maturity often stipulate on sprouts; petioles and young leaves often sparsely to ± heavily pubescent with copper-colored (sometimes whitish) hairs; scales usually with a prominent glabrate area on apical third or half; mature capsules 4–6.5 (–7.5) mm long, smooth (except usually in wrinkled beak), dehiscing before June 15 (or as late as July 1 in northern Michigan or late seasons).

26. Leaves somewhat whitened to strongly glaucous beneath, acute or short-acuminate, estipulate; petioles and young leaves glabrous; scales of both staminate and pistillate aments usually ± pubescent to their tips; mature capsules (6–) 7.5–10 (–12) mm long, ± granular-roughened or wrinkled throughout, dehiscing after July 1 (occasionally June 15–July 1 in southern Michigan or in dry or early seasons).

27. Scales of aments yellowish, pilose at base and margins, glabrate on back, deciduous before ripening of capsules; flowers tending to be spaced in whorls, in rather slender, lax aments; capsules glabrous; stamens 2–7; plants native or introduced trees (when full-grown), with lanceolate, usually attenuate leaves, these glabrous to somewhat silky at maturity.

31. Surface of leaves whitened or glaucous beneath, glabrous or retaining pubescence. [Note: If leaves are rather coarsely crenate-toothed and the specimen does not work well here, try couplet 16. Furthermore, all five species below are included in the supplementary keys to flowering material (although only the last two normally have aments before the leaves); those keys may be helpful to consult for material with aments].

33. Leaf blades ± rounded (occasionally acute) to cordate at base, often over 1.5 cm wide at maturity, glabrous or retaining a little pubescence along the midrib; capsules glabrous; stipules often present and conspicuous, especially on sprouts, ovate to reniform; young unfolding leaves at tips of branchlets usually reddish.

34. Foliage with a definite “balsamic” (almost cosmetic or spicy) odor, which is usually persistent long after drying; mature leaves completely glabrous; year-old branchlets red and shiny, smooth and glabrous (the current year’s branchlets glabrous to somewhat puberulent when very young); mature capsules on stalks 2–4 mm long, subtended by rather uniformly light brown scales.

35. Young unfolding leaf blades at tips of branchlets often reddish, glabrous or usually with some copper-colored hairs mixed with whitish; mature leaves thick, usually very glaucous beneath; styles ca. 0.6–1.5 mm long; young staminate aments flowering from apex to base.

36. Mature leaves glabrous or nearly so above (except for puberulent midrib), the underside ± silky, usually rather densely so, with the main lateral veins prominently rib-like; young leaves densely silky, without any coppery hairs; year-old branchlets puberulent (or sometimes glabrate), those of the current year usually densely so; capsules (2–) 3.5–6.5 mm long, rather plump (ovoid) and rounded at tip.

Do not try to use these keys on specimens either with very immature aments or with foliage leaves developed, as the species may not be included or the measurements may differ from those given here. (Developing leafy bracts may, however, occur on the flowering branchlets or “peduncles” in several species keyed below.)

These keys are only guides to typical material; extremes of variation may cause difficulty. It is always best to collect leaves from the same plant later in the season. Be sure to measure the most mature examples of any part described, and several of them. Stalks of the ovaries, in particular, may be shorter than the measurements given if the flowers are not fully developed. Measurements of styles are of the undivided portion. Measurements are based primarily on dry specimens, and anthers, in particular, may be slightly larger when fresh.

7. Ovaries up to 5.5 mm long; leaves always alternate; youngest branchlets and growth of current year (including bracts on flowering branchlets) just beginning at flowering time, finely silky-pubescent.

16. Bracts thinly silky or glabrous except at base, the margins without glands; aments flowering from base to apex or from middle to both ends. [The rare escape Salix viminalis will run here, as well as some puberulent-branched specimens of S. petiolaris and small-anthered specimens of S. eriocephala. Pistillate material and/or leaves are necessary to distinguish these with certainty.]